1. Field of the Invention
This invention relates to new and useful improvements in multieffect wiped film rotating evaporators which can be used in mechanically driven vapor compression distillation and in thermally driven multieffect distillation processes that can be employed in wide varieties of applications such as the desalting of saline waters, the recovery of distilled water from gray and brown waste waters, the concentration of toxic liquid waste, the concentration of milk, the production of alcohol, and other distillation separation processes.
This invention also relates to the use of the module in life support systems for the recovery of distilled water from waste water in partial gravity conditions in isolated environments such as space crafts and future Lunar and Martian colonies.
2. Description of the Prior Art
A wide variety of single and multieffect rotating evaporators are known for accomplishing these goals. For example, U.S. Pat. No. 2,703,310 to Kretchmar (1955) shows a vacuum distillation system which includes rotating flat disks and a series of stationary condensers where the disks dip into a pool of a heated solution and condensation occurs on the outside surface of the condenser tubes resulting in heat transfer factor of the same magnitude as in conventional distillers.
U.S. Pat. No. 2,894,879 to Hickman (1959) shows a multieffect distillation apparatus in which rotating conical disk pairs forming cavities are employed. The feed to be evaporated is sprayed on the inside surfaces of the disk pairs and the heating vapor is condensed on the outside surfaces of the disk pairs, the unevaporated feed collected in the periphery is withdrawn by stationary scoops inside the cavities. Here, dry spots that form on the evaporation side of the disk will prevent the feed from reaching the areas behind the dry spots rendering those portions of the area of the disk ineffective. Also sediments in the feed could deposit in the periphery and clog the scoops.
U.S. Pat. No. 3,764,483 (1973) to Tleimat discloses a single effect rotating disk evaporator in which the rotor consists of rotating disk pairs forming cavities where the feed is spread by means of stationary wipers into a very thin film on the outside surfaces of the disk pairs and the heating vapor condenses on the inside surfaces of the disk pairs. The condensate formed inside the cavities is withdrawn by means of stationary scoops between the disks. Data obtained using seawater feed showed the heat transfer coefficient to be an order of magnitude higher than that in conventional distillation systems (about 5000 compared to 500 Btu/hr ft.sup.2 F) with no scale deposition on the disks. However, its use in multieffect distillation systems requires the connection of multiple units in series with its associated problems of heat losses, intereffect leakages, connecting piping, and cost.
U.S. Pat. No. 3,890,205 to Schnitzer (1975) shows an integral vapor compression apparatus in which one or more disks, rotating at high speed, are used as the heat transfer surface between boiling and condensing fluids. This embodiment is susceptible to the formation of dry spots on the evaporation side of the disk and may be limited to small capacity applications.
U.S. Pat. No. 4,504,361 to Tkac et al (1985) shows flat stationary surfaces where the heating medium is applied on one side and the feed to be distilled is spread or scraped on the other side by means of rotating wipers/scrapers. In this embodiment the heat transfer factor would be much lower due to the absence of centrifugal force on the condensation side that is present in rotating disk systems.
U.S. Pat. No. 4,586,985 to Ciocca et al (1986) shows a multieffect still which consists of rotating disks that form chambers in which condensation and evaporation occur. The heating steam enters the first chamber and condenses on the left side of the second disk and saline water enters the second chamber and is deposited on the right side of the second disk where part of it evaporates and the other part is fed to the third chamber. The resulting vapor condenses on the left side of the third disk to evaporate an equivalent amount of saline water feed from the right side of the third disk and so on to the last effect. The resulting vapor from the last effect condenses on the left side of the last disk which is being cooled by saline water on its right side. Again, this embodiment does not prevent the formation of dry spots on the evaporation side and is limited in capacity due to the fact that the heat transfer area per effect is limited to the surface area of each individual disk.
U.S. Pat. No. 4,707,220 to Feres (1987) shows a rotating evaporator consisting of pairs of conical disks connected together inside a drum to form the rotor. In this embodiment, evaporation occurs on the inside surfaces of the disk pairs and condensation of the heating vapor occurs on the outside surfaces of the disk pairs. The feed enters the cavity between the first pair of rotating disks and is spread on its inside surfaces. Evaporation occurs by condensing the heating vapor, from an external source, on the outside surfaces of the disk pairs. The unevaporated portion of the feed at the outside periphery of the cavity is picked up by a stationary scoop located between the disk pair and is fed into the second disk pair where further evaporation occurs. The more concentrated solution is similarly fed into the third disk pair and so on to the last disk pair where the unevaporated portion is taken out as residue. The condensate resulting from the heating vapor is collected on the inside surface of the rotating drum and is picked up by a stationary scoop and taken out as distillate. Again, this embodiment does not prevent the formation of dry spots on the inside surfaces of the disks and sediments present in the feed could cause clogging of the scoops. Its capacity is also limited because the heat transfer area per effect is governed by the surface area of each pair of disks.